1. Field of the Invention
The present invention relates generally to filter cartridges, and in particular to filter cartridges including a plurality of filter cells spaced apart by external cell separators.
2. Description of Related Art
The present invention is an improvement in filter cartridges, and particularly filter cartridges of the depth type, in which a cartridge comprises a plurality of individual filter cells. The cells are mounted about a central hollow core. Openings are formed in the core which communicate with the interiors of the filter cells. Fluid to be filtered flows from the exterior of the cells through the filter media and exits through the core.
Certain depth filter cartridges, such as those sold under the trademark ZETA PLUS by the AMF Cuno Microfiltration Products Division, Meriden, Conn., employ an internal thin perforated polypropylene disc within the cartridge to keep the media separated during use. The disc preferably includes upper and lower ribs of varying thickness to maintain the dished shape of the media and the internal space between them. There also may be a seal ring between two cells and adjacent the core to prevent bypass of non-filtered solution from one cell to each other.
It has been found that when filtering fluids, differential pressure and filter-cake buildup on the filter media can cause distortion of the cell and, in addition, minor swelling that can restrict the flow channel between cells, causing decrease in flow per unit of differential pressure.
The filter cells in such cartridge can also become weakened by heat and distort or even collapse when used to filter hot liquids, for example over about 40.degree. C. Heat may also cause the cell shape to be distorted horizontally, causing contact between the cells or breakage of the seal at all or part of the outer edge of the cell. Also, the media themselves may be broken by heat.
These problems may also occur due to use or handling. If they occur, only part of the media may be effectively used for filtration.
These problems have been found to be non-uniform from cell to cell, because it is difficult to provide perfectly uniform media and internal separators and to assemble these in a consistent fashion.
Further, when filtering certain organic liquids, e.g. hexane, and after sterilization by steam, the media may swell, causing contact between the cells, leading to ineffective use of the filtration area.
Prior art filter structures have not satisfactorily addressed these concerns. For example, U.S. Pat. No. 3,666,107 discloses a filter cartridge comprising a plurality of filter elements each consisting of two dished diaphragm sections of filter material, each section having attached thereto one or more circumferential or radial stiffening members or ribs. Such construction is believed to be difficult to manufacture and would impair to some extent the usable surface area of the filter elements.
A prior art filter cartridge having wafer-like elements but with no additional support therefor is seen in U.S. Pat. No. 2,966,269. This patent discloses a filter assembly including filter cartridges, each constituting a stack of wafers, each wafer being formed from a pair of opposed dished annular discs of filter paper or the like. The discs are secured around their outer peripheral edges, for example by an adhesive, and are spaced apart at their inner peripheral edges by annular spacers between the discs, i.e. within the wafers. The annular spacers have apertures opening into the interior of the wafer. The wafers are stacked together in generally cylindrical form and held together between end plates by tie rods.
Other filter cartridge structures are disclosed in U.S. Pat. Nos. 3,165,471; 4,,094,793; 1,102,685; 1,742,919; 2,411,341; 3,178,028; and 4,048,073.